A passive optical network (PON) system is known in which an optical line terminal (OLT) and a plurality of optical network units (ONUs) are connected to each other by a coupler.
As the distance of transmission performed in the PON system becomes longer, it is desired to provide an optical amplifier along a transmission path. In addition, as an optical amplifier for bidirectional optical transmission, a configuration using a fully circulating type four-terminal optical circulator is known (for example, refer to Japanese Laid-open Patent Publication No. 11-274625).
In addition, a technique for detecting a failure in the transmission path of the PON system by transmitting monitoring light having a certain wavelength along the transmission path of the PON system and monitoring reflected light of the monitoring light is known.